This invention pertains to liquid crystal displays. More particularly this invention pertains to reflective liquid crystal displays.
Liquid crystal displays (LCD) are used in a variety of electronic devices including pagers, and portable digital assistants. Liquid crystal displays offer the advantages of compactness and low power consumption. Liquid crystal displays include an array of separately controllable display areas termed pixels. Each pixel includes electrodes for establishing a localized electric field that locally effects the state of a liquid crystal material. Propagation of light through the each pixel is in turn affected by the state of the liquid crystal material. Spatial modulation of light is achieved by applying different signals to the electrodes associated with different pixels of a display.
One variety of liquid crystal display is the reflective type. Reflective liquid crystal displays spatially modulate light by selectively (e.g., according to an image pattern) reflecting incident ambient light. The brightness of reflective liquid crystal displays is determined by the brightness of the ambient light, and the reflectivity of the display. In addition to brightness another important measure of performance of liquid crystal displays that is related to brightness is contrast. Contrast is the difference between the brightness of a display area (pixel) that is set to a non-reflecting (dark) state, and the brightness of a display area that is set to a reflecting (bright) state. Contrast of a display is diminished by reflections off of surfaces, and interfaces within the display that are located in front of the liquid crystal material. Such reflections are not spatially modulated and therefore increase the brightness of the entire display area including areas that are meant to be turned off. The latter problem is somewhat aggravated by the fact users tend to orient reflective liquid display at an angle close to the angle at which specular reflection of light from an ambient light source (e.g., fluorescent lamp, the sun) reaches their eyes. Unfortunately, orienting a liquid crystal display at an angle which leads to specular reflection of source light into a user""s eyes leads increases the intensity of both spatially modulated, and unmodulated light reaching the user""s eyes. In order to achieve acceptable contrast a user must precisely adjust the orientation of the display to an angle at which the spatially modulated light predominates over unmodulated light. The range of solid angle of orientation in which the display can be viewed (termed the viewing zone) may be quite limited, such that the user must be careful in maintaining the orientation of the display.
Published PCT application WO 98/32047 that is assigned to the assignee of the present invention teaches a liquid crystal display that includes a transmissive holographic element that deflects light so as to provide angular separation between light that has passed through a liquid crystal material layer (modulated light) and light that is reflected by the holographic without passing through the liquid crystal material layer (unmodulated light). The angular separation between the modulated and unmodulated light effectively increases the contrast of the display by allowing a user to view the modulated light without viewing the unmodulated light. A certain amount of reflection of spatially unmodulated light occurs at layer interfaces between the transmissive holographic element and the liquid crystal material. Such unmodulated reflection tends to decrease the contrast of the display
What is needed is a liquid crystal display that increases the intensity of spatially modulated reflected light, relative to the amount of unmodulated reflected light.
What is needed is a display that provides increased contrast.
What is needed is a needed is a system that provides a viewing zone of substantial angular extent.